Limited energy resources and the growing demand to decrease greenhouse gas emissions have intensified research of carbon-free energy sources. Batteries that store high-energy densities will play a large role in implementation of green energy technologies and non-petroleum vehicular mobility. To date, rechargeable Li-ion batteries offer the highest energy density of any battery technology, and are expected to provide a solution for our future energy-storage requirements. Unfortunately, Li-ion batteries have a number of limitations, such as capacity loss over time during long-term cycling due to phase transitions leading to detrimental volume changes in the electrode materials. This can cause local atom rearrangements that block the diffusion of Li ions, leading to high over potentials and loss of capacity. In addition, Li-ion batteries that have been charged quickly can form dendritic Li deposition at the commercial graphite anode and can create a safety problem that in the worst scenario could cause thermal runaway, cell rupture and explosion. Use of chemically inert anode materials such as metal oxides with lithiation voltages positive of Li-deposition compared to carbon can address these issues and improve safety of Li-ion battery operation.
High-performance battery materials are critical for the development of new alternative energy storage systems. While Li-ion batteries are a mature technology for energy storage, disadvantages include cost, Li supply, safety, reliability and stability. Moreover, the electrolyte stability over time is additional major concern for long-term operation and advanced applications. Thus, the discovery, research and development of new transporting ions that can provide an alternative choice to Li batteries are essential for further advancement of energy storage materials. Sodium-based batteries are attractive due to the promise of low cost associated with the abundance of sodium, and enhanced stability of non-aqueous battery electrolytes due to the lower operating voltages. However, lower voltage leads to insufficient energy density, thus cathode materials for Na batteries must possess high-capacities. Since the ionic volume of sodium is almost twice that of lithium, unique crystalline structures have to be used to accommodate incorporation of large ions.
Titanium dioxide is one of the few metal oxide materials that intercalates Li ions at reasonably low voltage (approximately 1.2 V vs. Li/Li+) and is suitable as a battery anode material. The first attempts of using TiO2 for a durable and safe electrode material were focused on microcrystalline TiO2 materials such as rutile, anatase, and TiO2(B). These electrodes materials showed moderate specific capacities (e.g., maximum Li uptake of 0.5Li/Ti for anatase and TiO2(B), and no activity for rutile) due to the limited room temperature reactivity and conductivity at microscale. Recently, the idea of using TiO2 electrodes has been revisited with the consideration that nanosize morphologies provide enhanced intercalation kinetics and large surface area associated with high accessibility of transporting ions. Reversible capacities with stoichiometries up to about approximately 0.5-0.7Li/Ti have been demonstrate; however, repetitive cycling caused loss of capacity independent of the crystalline modification. It was expected that all TiO2-based electrodes will have an intrinsic capacity limitation, because of a low number of crystallographic sites, and their electronically insulating structure. Both experimental and theoretical studies of intercalation of Li ions in crystalline polymorphs of TiO2 show that high lithium content can be obtained exclusively at elevated temperatures raising concerns from the application point of view.
Vanadium pentoxide (V2O5) has been intensively studied as the positive electrode (anode) material for lithium ion batteries. In previous studies, various fabrication methods were used: sputtering, thermal evaporation, thermal decomposition, electrophoretic deposition and many chemical routes, such as hydrothermal synthesis, and sol-gel method. It has been reported that chemical composition, crystal structure and crystallinity of V2O5 may have pivotal roles in lithium ion intercalation capacity and cycling stability. To date, the practical application of vanadium oxides in lithium ion batteries was still limited due to the poor cyclic stability and undesirable phase transitions to inactive materials.
In contrast to lithium batteries, a report of Liu et al. considered employment of vanadium oxide by insertion/deinsertion of sodium-ion into NaV6O15 nanorods. Even though the morphology of the reported material could be retained, the initial discharge capacity of 142 mAhg−1 substantially decreased by cycling at higher current densities, which led to poor overall performance. In order to explore full potential in utilization of nanostructured V2O5 electrodes for sodium batteries a detailed fundamental insight is necessary.
There is an ongoing need for new, more efficient, electrode materials for lithium-ion and sodium-ion batteries. The present invention addresses this need.